The present invention relates to a novel process for preparing a bulk powder useful as a raw material for an oxide superconductor, and more particularly to a process for preparing ultra-fine particles for forming a copper-barium-lanthanoid rare earth metal compound oxide (Cu-Ba-Ln oxide) superconductor, wherein barium carbonate BaCO.sub.3 is not formed as an intermediate.
It was found that a 1:2:3 formulation of copper-barium-yttrium (yttrium being one of lanthanoid rare earth metals) metal compound oxide (YBCO) having an oxygen-deficient layered perovskite structure was a superconductor showing a superconducting transition temperature exceeding the liquid-nitrogen temperature. Since then, the YBCO has been expected to be practically used for various uses such as semiconductor devices using a superconductor, superconducting magnet, energy storage system, magnetic shield and sensors, because the liquid-nitrogen which is inexpensive and high in cooling efficiency can be used as a coolant. So, energetic studies of the YBCO superconductor have been made in various fields.
The above-mentioned Cu-Ba-Ln oxides such as YBCO are a type of ceramic. In case of using the polycrystalline sintered product as a superconductor material, the same improvements as required in common ceramics are required. That is, in order to secure the performance as the superconductor, it is required that the secondary phase as impurities is not deposited in the crystal grain boundaries of the superconducting oxide formed by sintering; the density of the sintered product is approached to a true density as much as possible; and the grains are uniform and fine in grain size. Further, the oxide is required for performing the superconducting properties to have a very limited composition and crystal structure. For example, it is necessary that the ratio of yttrium, barrium and copper (Y/Ba/Cu atomic ratio) is close to 1/2/3, the number of oxygen atoms to (Y.sub.1 Ba.sub.2 Cu.sub.3) is from about 6.7 to 7.0, in other words, the atomic ratio of oxygen to yttrium is about 6.7 to 7 when the Y/Ba/Cu atomic ratio is approximately 1/2/3, and the crystal structure is an orthorhombic structure. When the above requirement composition and crystal structure of the oxide is not satisfied, some of the superconducting properties such as a superconducting transition temperature, a critical current density and a critical magnetic field is consequently lowered. Accordingly, it has been recognized that the sintering conditions and the conditions for preparing the raw material powder for oxide superconductors are important. In other words, it is desired that the bulk powder used as the raw material for the oxide superconductor has a homogenity in metallic ratio (hereinafter referred to as "are excellent in stoichiometry") and are fine in particle size.
According to solid-state reaction known as a usual way for synthesizing the bulk powder for oxide superconductors, a mixture of copper oxide, barium oxide and a rare earth metal oxide is calcined at a high temperature, e.g. about 950.degree. C., for a long time and pulverized, and the calcining and pulverizing procedure must be repeated several times for giving the bulk powder having uniform phase as measured by X-ray analysis. Accordingly, the grain growth occurs, as the result the obtained powder has a particle size as large as several tens of .mu.m. So, problems that the powder is poor in sintering property are still left.
Therefore, for providing a bulk powder for the metal oxide superconductors which is more excellent in stoichiometry and are more fine and uniform in particle size than the powder prepared by the solid-state reaction and for providing the powder by calcining at a lower temperature for a shorter period of time, many studies have been made. As such processes, there have been proposed some methods such as an oxalic acid coprecipitation method, a carbonate coprecipitation method and a gel decomposition method.
However, these processes have the defects as mentioned below respectively in addition to the common problem that there is a step using water, by which the oxide superconductor is apt to be easily damaged.
The oxalic acid coprecipitation method is a method wherein an aqueous solution of oxalic acid or oxalic acid compound is added dropwise to a solution of copper nitrate, barium nitrate and a rare earth metal nitrate in water or an ethyl alcohol-water mixed solvent to give a coprecipitate. In the case of this method, since the pH of the reaction system must be delicately controlled for maintaining the required stoichiometry, it is very difficult to operate the reaction and other additives are required for adjusting the pH.
The carbonate coprecipitation method wherein an aqueous solution of copper nitrate, barium nitrate and a rare earth metal nitrate is neutralized with an alkali metal carbonate such as potassium carbonate to coprecipitate, requires careful washing of the obtained coprecipitate for removing the alkali metal ion remaining therein. Nevertheless, there is a case that the alkali metal remains in the obtained product.
The gel decomposition method is a method wherein citric acid and ethylene glycol are added to an aqueous solution of copper nitrate, barium nitrate and a rare earth metal nitrate and the mixture is heated to give a gelatinous substrate, then the substrate is thermally decomposed. According to this method, carbon may remain in the obtained powder.
Furthermore, in case of the above three methods, the desired product cannot be directly obtained by calcination but can be obtained through formation of a mixture of barium carbonate, copper oxide and a rare earth metal oxide which is the same one as in the solid-state reaction. Therefore, since barium carbonate comparatively stable to heat is present in the calcining step, the synthesis temperature becomes inevitably high, and a homogeneous phase cannot be easily obtained due to diffusion rate-determig step of carbon dioxide gas generating in case of calcining a large amount of the powder at once or calcining that in a vessel deep in bottom.
An object of the present invention is to solve the above-mentioned defects in conventional techniques and to provide a process for preparing a bulk powder for an oxide superconductor without forming barium carbonate as an intermediate product, whereby the temperature of YBCO formation is lowered, thus a fine and good stoichiometric powder is obtained.
This and other objects of the present invention will become apparent from the description hereinafter.